Keep in mind that REL is about quality of construction/functioning....NOT it's chance of jamming. I think that is where most people's confusion comes from.
I read that M60s were notorious in vietnam for barrel melts due to high ROF
https://www.military.com/video/guns/mac ... 0909933001 30 seconds into firing the barrel is glowing ...and pipe bombs and zip guns are shoddy/hurried manufacture. Which is why almost everything is REL 5. It is not about an AK being able to take mud/dirt better than an M16, it is purely about the construction/functioning.
pg 91 player's manual "Each point of damage from
pushing rolls or outside force will reduce the reliability
rating by 1. When it reaches zero, the item is broken and
can no longer be used. A typical piece of sturdy, mechanical
gear in good condition has a reliability rating of 5, but
delicate or badly built gear can start with a lower rating.
If no hit reliability is listed, assume it to be 5."
So in the M60's case of REL 4...the more you push...the more you are "melting" the barrel. etc.
Vcutter points out the good rule on 91 about maintenance and care....that if not done, the REL will drop. I think the mechanic would then be that M16s need more maint to prevent REL from decreasing than AKs. comparison:
https://www.military-today.com/firearms/m16_vs_ak47.htm
Differences of chance of jams are not in the rules from what I can see.